Two highly invasive hawkweeds, Hieracium aurantiacum and H. praealtum L., have established in the Australian Alps. Our experiments aimed to provide a better understanding of the seed ecology of these species, essential if successful eradication strategies are to be developed. Results indicated that viable seeds are produced in large numbers. Seeds germinate spontaneously and synchronously with light and moisture and are killed when exposed to temperatures above 110°C for 150 s. No seedlings of H. aurantiacum emerged from soil samples taken from a chronosequence of known infestations, but some H. praealtum seedlings emerged from soil sampled from sites where plants flowered the preceding summer. Germination was greatly reduced for both species under darkness, and few seedlings emerged when buried. Where herbicides were applied at a late stage of fruit development, hawkweeds continued to produce viable seeds, albeit in reduced numbers. Overall, our findings indicate that both species produce a large amount of seed, little of which enters the long-term soil seed bank. This, and the limited ability of buried seeds to germinate and/or emerge, means that Hieracium control programs should emphasise short-term (1–2-year) monitoring of treated sites and stress the importance of reducing the seed set.